A microscope of the species is known, for example, from U.S. Pat. No. 5,303,082. This known stereomicroscope comprises a basic body on or in which microscope objectives (which in some cases comprise components used on a shared basis), two eyepieces, and a number of beam splitters are arranged. The beam splitters are provided in the beam paths between the microscope objectives and the eyepieces. The beam splitters serve on the one band to reflect light from illuminating light sources coaxially into the beam path in order to illuminate the specimen being observed, and on the other hand to create simultaneous observation capabilities.
Also known, from German Pat. No. 36 08 242, is a slit lamp microscope for ophthalmological applications in which a beam splitter is introduced into the beam path between the microscope objective and the eyepiece in such a way that an image is reflected into the beam path going to the eyepiece. This image, which for example can be an angiography image, is overlaid on the “real” microscope image so that the viewer can compare the microscope image to the angiography image and optionally superimpose them.
It is true of all the known microscopes of the species that the beam splitter or splitters are immovably introduced into the beam path, i.e. mounted immovably on the basic body or chassis of the microscope or in a beam splitter housing.
This configuration of the known microscopes of the species has a number of disadvantages:                1. The beam splitters introduced immovably into the beam path or paths unnecessarily reduce the brightness of the microscope image when there is no need for reflection into or out of the respective beam path.        2. The arrangement of the respective beam splitters defines the direction in which any reflection into or out of the beam paths is accomplished.        3. The immovably installed beam splitters do not allow any possibility of variation, for example in terms of the ratio at which the incoming light intensity is split, the type of beam splatters used, or the like.        4. The two beam paths of the stereomicroscope of the species known from U.S. Pat. No. 5,303,082, and of other stereomicroscopes, are configured symmetrically; it is therefore not possible to reflect the microscope image out in the one beam path and reflect into the other beam path an image that is overlaid on the microscope image.        
In addition, reference is explicitly made to the aforementioned existing art for explanation of any details not described here.